1. Field of the Invention
The present invention relates to a ball screw which is used in various industrial machines and acts to convert a rectilinear motion to a rotary motion or vice versa.
2. Description of the Related Art
A conventional ball screw comprises a threaded rod and at least one ball nut. The threaded rod has a ball-rolling groove formed spirally with a given lead. Ball bearings are constrained to roll in this groove. The ball nut has a load ball groove and a ball return passage. The load ball groove is formed spirally with the same lead as the ball-rolling groove. The ball return passage permits the ball bearings to circulate from one end of the load ball groove to the other end. It is possible to take a rectilinear motion from one of the threaded rod and the ball nut by imparting a rotary motion to the other.
During the use of this ball screw, it is customary to impart a pre-load to the ball nut to eliminate any axial gap, to increase the rigidity against axial load for avoiding backlash, and to improve the accuracy of the feeding of rectilinear motion.
A typical method of imparting a pre-load to the ball screw uses two ball nuts and consists in imparting the pre-load to the screw while exerting a tensile force or compressive force between these two ball nuts. This is a so-called double-nut method. In particular, this method is classified into a constant position pre-loading method in which a spacer having a thickness corresponding to the amount of the pre-load is inserted between one pair of ball nuts and a constant pressure pre-loading method in which a pre-load is imparted by making use of the resilient force of a spring sandwiched between a pair of ball nuts.
In the former constant position pre-loading method, the two ball nuts are tightened together rigidly via the spacer. Therefore, if the lead of the threaded rod or the diameter of the rod contains an error, then the ball bearings are resiliently deformed nonuniformly. Consequently, the pre-load is nonuniform among the ball bearings. As a result, the torques applied to the ball nuts are different from each other. In the latter constant pressure pre-loading method, such a problem does not take place, because the amount of resilient deformation of the ball bearings is maintained constant at all times by the action of the spring. However, a decrease in the rigidity is unavoidable. Furthermore, it is difficult to mount the spring in such a way that an appropriate pre-load is obtained. Also, in either method, an adequate pre-load is not imparted where the ball nuts rotate relative to each other. For this reason, it is required that the two ball nuts be coupled together without permitting them to rotate relative to each other. Therefore, the number of components is increased. Also, the number of assembly steps is increased. Another problem is that a smooth rotary motion cannot be obtained unless the axes of the two ball nuts agree strictly.
In these situations, a so-called offset pre-loading method has been proposed to impart a pre-load with a single ball nut. Specifically, the ball nut is provided with a spiral load groove receiving ball bearings. The lead of this groove is increased and decreased at certain locations, depending on the amount of the pre-load, so that a ball contact structure similar to that derived by the above-described constant position pre-loading method is obtained. In this method, the pre-load is given with the single ball nut. The total length of the nut is reduced compared with that of the double nut structure. Also, the weight is reduced. Furthermore, a miniaturization can be realized. In addition, less labor is needed to assemble the ball screw.
Nonetheless, even in this offset pre-loading method, if the lead of the threaded rod or the diameter of the rod contains an error due to machining error, then the torque applied to the ball nut is nonuniform, in the same way as in the constant position pre-loading method. Additionally, any radial gap produces an axial gap. This deteriorates the feeding accuracy.
In this manner, the prior art pre-loading methods have problems concerning the manufacture of the ball screw and regarding the accuracy of motion and fail to satisfy all the users and manufacturers of these ball screws.